Lose 9 pounds and be more fertile.

Shedding just 5 percent of their body weight (about 9 pounds) caused obese women to have a 19 percent jump in blood flow to the womb, preliminary research from the University of Sheffield shows. This result greatly improves ovulation, raising the chance of pregnancy.

Lose 11 pounds and ease your aches.

Women who weighed 150 pounds and shed about 11 pounds cut their risk for osteoarthritis of the knee by 50 percent, the American College of Physicians reports. "Each pound of excess weight translates to 3 to 6 pounds of extra weight on your knees when walking or jogging, a force they can't withstand," says lead researcher David T. Felson, M.D. "Losing weight can prevent this damage."

Lose 15 pounds and protect your heart.

Overweight people who lost 15 or more pounds reduced their risk for hypertension by as much as 28 percent. "Weight loss lowers pressure within the blood vessels, putting less strain on your heart," explains Lynn Moore, D.Sc., associate professor of medicine at Boston University School of Medicine.

Lose 20 pounds and avoid breast cancer.

Overweight women who lost 20 pounds reduced their risk by 19 percent, a study from the University of Minnesota at Minneapolis suggests. Fat cells produce excess estrogen, which may play a role in the proliferation of cancer cells.

Or gain a little weight and live longer!

Underweight women whose body-mass index is below 18.5 have a higher risk than overweight women of dying from noncancer, non?heart disease causes such as pneumonia, injury or Parkinson's disease, the Journal of the American Medical Association notes. If you're too thin, you may not have the nutrient stores to help your body heal. Check Self.com's BMI calculator to find out if you're in the healthy range.